Quay County (/ˈkweɪ/) is a county in the state of New Mexico. As of the 2020 U.S. census, the population was 8,746. Its county seat is Tucumcari. The county was named for Pennsylvania senator Matthew Quay, who supported statehood for New Mexico. Its eastern border is the Texas state line, approximately 103.04 degrees west longitude.

Modern-day Quay County was once home to a massive inland sea, known as the Western Interior Seaway. Like the rest of New Mexico, Quay County has a strong paleontological history and dinosaur fossils have been found in the area.

The earliest known evidence of human activity dates back to roughly 9500 BC, when the Clovis culture emerged in the region. Around 900 AD, the Ancestral Puebloan culture emerged in the region, and is believed to have lasted until roughly 1400 AD. While settlement was largely in the Four Corners region of the state, artifacts have been discovered in Quay County as well, and the area is believed to have been a trade route or a temporary hunting site. By the 1500s, Native American tribes such as the Apache and Comanche had emerged, using the area as hunting grounds. Around this time, Spanish explorers began scouting the area, but did not settle in Quay County or establish any missions or trading posts.

European settlements emerged in Quay County around the 17th century, although they never gained any major size or importance. Tensions between settlers and Native American tribes were common due to Comanchero interference, and settlement remained sparse until the Americans obtained control of the New Mexico Territory and established Fort Bascom in 1863. As Native Americans were relocated following the Apache Wars, settlers steadily moved into the area and the cattle industry spilled over from Texas, becoming the dominant economic driver of the region. Prior to the 1860s, most settlers had been Hispano ranchers and shepherds, but by 1899 the region was full of Anglo-American homesteaders.

In 1902, the railroad came to Quay County. Tucumcari and Logan were founded as railroad camps for the Chicago, Rock Island and Pacific Railroad, and Quay County was officially split off from neighboring Guadalupe County and incorporated in 1903. By 1913, Tucumcari had become a boomtown. Around this time, harsh weather and tensions with ranchers caused most of the homesteaders to abandon the region, and by the 1920s Quay County was predominantly home to ranchers and new urban residents.

Tucumcari grew so quickly during this time that it was one of few areas in the United States to not be significantly affected by the Great Depression. Following World War I, returning servicemembers proved a boon to the economy and a building boom began that lasted for years.

In 1926, U.S. Route 66 was established, quickly surpassing the railroad in terms of importance. Quay County was a major thoroughfare, and the population of the area boomed as the economy quickly shifted towards the service sector. By the 1950s, Tucumcari was referred to as "Little Las Vegas" and was one of the most important commercial centers of eastern New Mexico.

With the advent of the Interstate Highway System in the 1960s, Quay County quickly declined in importance as I-40 (which served only Tucumcari) replaced Route 66. The population of Quay County has since declined steadily, and it is now a rural area with only one large commercial center at Tucumcari.